This invention relates to torsional vibration dampers such as twin mass flywheels and in particular such flywheels used to connect an internal combustion engine with a gearbox on a motor vehicle.
In, for example, the Applicant""s earlier granted patents GB 2229793 and 2282868 and pending applications PCT/GB95/01976 and PCT/GB97/00361 there are described torsional vibration dampers in the form of twin mass flywheels in which an input element and an output element are relatively rotatable against the action of a damping means which includes a plurality of linkages each linkage including one or more link members which act as bob weights to provide a speed dependant damping on the relative rotation of the flywheel elements. The Applicant""s earlier patent applications GB-A-2220464 and PCT/GB97/30302 also disclose torsional vibration dampers used in combination with a fluid coupling in which the relative rotation of input and output elements of the damper is again controlled by a plurality of such linkages.
Such vibration dampers can be prone to problems associated with tilting of the input element out of its normal plane of rotation due to flexing of the crankshaft of the associated engine. Such tilting can impose undesirable loadings in the linkages which interconnect the elements.
It is an object of the present invention to provide a vibration damper which at least partially mitigates the above problem.
Thus according to the present invention there is provided a torsional vibration damper comprising an input element and an output element which are relatively rotatable against the action of a damping means which includes a plurality of linkages, each linkage comprising one or more links mounted on one element via pivot pins and arranged to extend generally radially in use and connected with the other element by one or more additional links, damper being characterised in that the pivot pins of the generally radially extending links are supported from said one damper element via spaced bearings in said one damper element.
Such a spaced bearing arrangement is applicable to all the torsional vibration dampers disclosed in the above referred to earlier patents and applications when used as a twin mass flywheel where the input and output elements comprise input and output flywheel masses respectively or when used as a damper in which the input and output elements are of relatively light mass and are connected in a drive-line either as a separate unit or in combination with a fluid coupling as discussed above.
Such a torsional vibration damper may also include generally radially extending links which have an intermediate portion designed to flex in an axial sense to accommodate forces applied to the link via the associated additional link as a result of relative tilting of the damper elements.
The pins which mount the generally radially extending links are preferably a clearance fit in the bearings, the level of clearance being sufficient to ensure that any thermal expansion of the pins during use of the flywheel is insufficient to cause the pins to bind in the bearings.
For example, when a mounting pin of 10 mm diameter is used a clearance of 0.1 m (100 microns) between the internal diameter of the bearings and the external diameter of the mounting pin is sufficient (i.e. a clearance of the order of 1% of pin diameter).
The bearings can be in the form of plain polymeric bushes (for example polyimide resin with a filler such as graphite). Many other alternative plain bearing bush materials could be used, for example, a composite material comprising a PTFE/lead overlay with a bronze impregnated interlayer and a metal backing layer.
The bearing bushes may be directly mounted in the appropriate damper element or may be mounted via a metal or plastics housing. Such housings may include bush retaining flanges and may be of greater axial extent than the damper element in which they are mounted in order to allow the use of longer bearing bushes which lowers bearing contact pressure. The bearing bushes and/or housings may act as heat insulators to shield the pins to some extent against the worst effects of thermal expansion.
In an alternative arrangement the bushes may be axially retained by flanges provided on the element.
In certain applications needle bearings can be used instead of plain bearing bushes.
The pin is preferably hardened and is an interference fit in the associated link which is made from softer material. Typically the link will be made from mild steel and the pin from hardened steel with ground end portions which engage in the bearing bushes.
In an alternative arrangement the pin can be made from stainless steel and the end portions can be turned (i.e. produced by a lathe turning operation) to provide a helical patterned surface finish to assist bedding-in of the bearings.
If the pin is not made from harder material than the link the ends of the pin which engage the bearings are preferably of smaller diameter than the central portion of the pin which is an interference fit with the link in order to prevent damage to the pin end portions during assembly of the pivot.
In certain applications, particularly if needle roller bearings are employed, if may be desirable to hold grease captive within the bearing.
The invention also provides a torsional vibration damper comprising an input element and an output element which are relatively rotatable against the action of a damping means which includes a plurality of linkages, each linkage comprising one or more links mounted on one element via pivot pins and arranged to extend generally radially in use and connected with the other element by one or more additional links, the damper being characterised in that the generally radially extending links are mounted on their pivot pins via gimbal pins which allow axial pivoting of the links relative to the pins.